As so far known in the art, there is available a process for recording and reproducing electrostatic images wherein "image exposure" is carried out with the application of voltage between both the electrodes of a photosensitive member and electrostatic information recording medium which are located in opposite relation to each other, thereby forming an electrostatic latent image of high resolving power on electrostatic information recording medium.
Such an electrostatic image-recording process is illustrated in FIG. 1, in which electrostatic information recording medium is shown at 1, a photosensitive member at 2, a photoconductive layer support at 2a, an electrode of the photosensitive member at 2b, a photoconductive layer at 2c, an insulating layer at 1a, an electrode of electrostatic information recording medium at 1b, an insulating layer support at 1c and a power source at E.
Referring to FIG. 1,. exposure is carried out through the photosensitive member 2. The photosensitive member 2 is constructed by providing the transparent electrode 2b formed of a 1000 .ANG. thick ITO on the support 2a formed of a 1-mm thick glass and providing the photoconductive layer 2c of about 10 .mu.m in thickness on the electrode 2b. The electrostatic information recording medium 1 is located in opposite relation to the photosensitive member 2 through a gap of about 10 .mu.m. The electrostatic information recording medium 1 is formed by providing the A1 electrode 1b of 1000 .ANG. in thickness on the insulating layer support 1c by vapor deposition and providing the insulating layer 1a of 10 .mu.m in thickness on the electrode 1b.
As illustrated in FIG. 1a, electrostatic information recording medium 1 is first set with respect to the photosensitive member 2 through a gap of about 10 .mu.m.
Then, voltage is applied between the electrodes 2b and 1b from the power source E, as illustrated in FIG. 1a. In the dark, no change will take place between both the electrodes, because the photoconductor 2c is a high resistant. However, when a voltage higher than the Paschen's discharge voltage is impressed to the gap depending upon the magnitude of the applied voltage or leakage currents from the substrate electrode, discharge takes place through the gap, forming electrostatic charges corresponding to the discharge on electrostatic information recording medium. When the photoconductive layer 2c is irradiated with light incident from the photoconductive layer support 2a, it generates photocarriers (electrons, holes) at the irradiated region, and charges opposite in polarity to the electrode of electrostatic information recording medium store through the photoconductive layer 2c toward its surface. In the meantime, as the proportion of voltage assigned to the air gap exceeds the Paschen's discharge voltage, corona discharge or field emission takes place between the photoconductive layer 2c and the insulating layer 1a, so that charges can be extracted from the photoconductive 2c and accelerated by the electric field, causing accumulation of the charges on the insulating layer 1a.
After the completion of exposure, the photosensitive member and electrostatic information recording medium are short-circuited, as shown in FIG. 1c. It is noted that while voltage supply has been described as put off by opening the switch, this may also be achieved by short-circuiting both the electrodes. Then, the electrostatic information recording medium 1 is removed, as shown in FIG. 1d, to complete the formation of an electrostatic latent image. By putting on off the voltage applied in this way or, in other words, using a voltage shutter, it is possible to form an electrostatic latent image; it is possible to dispense with such a mechanical or optical shutter as used with ordinary cameras.
The photoconductive layer 2c is an electrically conductive layer which, upon irradiated with light, generates photocarriers (electrons, positive holes) at the irradiated region, allowing such carriers to migrate in the widthwise direction. This layer may be formed of inorganic or organic photoconductive materials or their hybrids.
The inorganic photosensitive materials used may include amorphous silicon, amorphous selenium, cadmium sulfide, zinc oxide and so on.
The organic photosensitive materials used are broken down into single-layer and function-separated types.
The single-layer type of photosensitive material comprises a mixture of a charge-generating substance with a charge transport substance. As the charge-generating type of substances likely to absorb light and generate charges, for instance, use may be made of azo pigments, bis-azo pigments, trisazo pigments, phthalocyanine pigments, perylene pigments, pyrylium dyes, cyanine dyes and methine dyes. As the charge transport type of substances well capable of transporting ionized charges, for instance, use may be made of hydrazones, pyrazolines, polyvinyl carbazoles, carbazoles, stilbenes, anthracenes, naphthalenes, triphenyl-methanes, azines, amines and aromatic amines.
Referring to the function-separated type of photosensitive material, the charge-generating substance is likely to absorb light but has the property of trapping photocarriers, whereas the charge transport substance is well capable of transporting charges but less capable of absorbing light. For that reason, both the substances are separated from each other to make much use of their individual properties. For use, charge-generating and charge transport layers may be laminated. As the substances forming the charge-generating layer, for instance, use may be made of azo pigments, bis-azo pigments, trisazo pigments, phthalocyanine pigments, acid xthanten dyes, cyanine dyes, styryl dyes, pyrylium dyes, perylene dyes, methine dyes, a-Se, a-Si, azulenium salt pigments and squalenium salt pigments. As the substances forming the charge transport layer, for instance, use may be made of hydrazones, pyrazolines, PVKs, carbzoles, oxazoles, triazoles, aromatic amines, amines, triphenylmethanes and polycyclic aromatic compounds.
Referring here to the nature of the carriers generated, it is known that in the case of the inorganic photosensitive material, the mobility .mu. is high but the life time .tau. is short, whereas in the case of the organic photosensitive material, the mobility .mu. is low but the life time .tau. is long, with the product of .mu..tau. being nearly on the same level in both the cases. The formation of an electrostatic latent image by the "exposure with the application of voltage" may be achieved even by a mechanical exposure shutter or voltage shutter alone. However, with the mechanical exposure shutter alone, voltage remains impressed between the photosensitive material and the electrostatic information recording medium, This in turn poses 8 a problem that even when exposure is not carried out, dark currents flow, giving rise to dark potential.
When only the voltage shutter is used with the organic photosensitive material, on the other hand, there is a problem that the quantity of exposure and the amount of charges vary with a voltage shutter time. This will be explained in detail with reference to FIG. 2.
FIG. 2 is a graph showing the amount of charges on electrostatic information recording medium at a constant light intensity but at varied voltage shutter times, say, 0.01 second, 0.1 second and 1 second. In the case of the inorganic photosensitive material which has a high carrier mobility, the amount of charges corresponds to the quantity of exposure even at varied voltage shutter times, as can be seen from a characteristic curve A. On the other hand, the use of the organic photosensitive material results in a phenomenon that even at the same amount of exposure, there is a difference in the quantity of charges between the voltage shutter times 0.01 second and 0.1 second, and 0.1 second and 1 second, as can be seen from characteristic curves B. This is because the organic photosensitive material has a low carrier mobility; the carriers generated by exposure disappear, since the voltage is cut off before they reach the charge-carrying medium. Thus, there is a problem that even at the same quantity of exposure, the image potential varies with a voltage shutter time.
When the photosensitive member and electrostatic information recording medium are short-circuited, as illustrated in FIG. 3, so as to cut off voltage supply, increased inverse voltage is induced between the photosensitive member and the charge-carrying medium, causing re-discharge in the inverse direction. This will now be explained in detail with reference to FIGS. 4 and 5.
The photosensitive member, gap and electrostatic information recording medium are all considered to be capacitors, each of given capacitance, and if the photosensitive member and electrostatic information recording medium have the same thickness, dielectric constant and area, then both will have an equal electrostatic capacitance. Also, given a gap of about 12-13 .mu.m between the photosensitive member and the electrostatic information recording medium, then the discharge voltage in the gap will be on the order of about 400V. For instance, now assume that the exposure with the application of voltage is carried out at an application voltage of 2000V. Then, the photosensitive member is made electrically conductive at the region exposed to light. Consequently, the overall "image exposure" system may be considered as an equivalent circuit in which, as illustrated in FIG. 4a, 400V and 1600V are applied to the capacitances C2 and C3 of the gap and electrostatic information recording medium, respectively. Similarly, the unexposed region may be taken as an equivalent circuit in which, as shown in FIG. 4b, 800V, 400V and 800V are applied to the capacitances C1, C2 and C3 of the photosensitive member, gap and electrostatic information recording medium, respectively.
Now consider potential distributions on the photosensitive member, and electrostatic information recording medium. For instance, it the electrode of the photosensitive member is defined as a reference position with a point P representing the end position of the gap, a point Q the end position of the gap and a point R the end position of the charge-carrying medium, then the distributions of potential on the exposed and unxposed regions are shown by P-Q-R in FIG. 5a and P-Q-R in FIG. 5b, respectively. This is because the photosensitive member is an electrical conductor.
When the photosensitive member and charge-carrying medium are short-circuited in such a state as shown in FIG. 5a, the point R is reduced to zero potential or a point R', and the point Q is reduced by the same potential difference or to a point Q', giving a potential distribution P-Q'-R'. Thus, a potential difference between P and Q', i.e., a voltage applied to the gap comes to 1600V.
This also holds for FIG. 5b; a potential difference between P and Q', i.e., a voltage applied to the gap comes to 1600V.
In consequence, the voltages applied to the respective capacitors are changed in state from FIGS. 4a and 4b to FIGS. 4c and 4d, respectively, in the equivalent circuit shown in FIG. 4. This poses a problem that an inverse voltage of 1600V that is much higher than the discharge voltage of 400V is so impressed on the gap that redischarge discharge can be instantaneously induced in the inverse direction, causing the recorded signals to fall into disarray and so rendering the dim image.
It is also well-known in the art to use a previously corona-charged, insulating layer film having an electrically conductive layer to form an electrostatic latent image thereon. To this end, exposure may be carried out while voltage is applied between the electrically conductive layer of the insulating layer film and the electrode of the associated photosensitive member, or both may be electrically short-circuited.
However, a problem with a conventional "image exposure with the application of voltage" process is that an external power source is needed to induce discharge by applying voltage between the photosensitive member and electrostatic information recording medium for exposure, only to render the system large in size and make the system likely to be affected by fluctuations in power source voltage.
If the previously corona-charged, insulating film is used, it may then be possible to dispense with using an external power source for exposure. Until now, however, nothing has been known about how to form latent images practically.
FIG. 6 is a diagrammatical sketch for illustrating a typical process, so far proposed, for recording electrostatic images with the use of a spacer.
Referring to FIG. 6, a photosensitive member 2--in which a transparent electrode layer 2b and a photoconductive layer 2c are successively laminated on the overall surface of a transparent substrate 2a--is located in opposite relation to electrostatic information recording medium 1--in which an electrode layer 1b and an insulating layer 1a are successively laminated on the overall surface of a substrate 1c--with a spacer 3 interposed therebetween. With voltage applied between both the electrode layers, the image exposure is carried out through, e.g. the photosensitive member 2. Then, the photoconductive layer 2c generates carriers at the exposed region and is made so electrically conductive there that discharge can take place at the exposed region between the photosensitive member and the electrostatic information recording medium, accumulating charges corresponding to the quantity of exposure on the insulating layer 1a and so forming an electrostatic latent image.
In the process for recording electrostatic images shown in FIG. 6, however, a variation of the gap length between the photosensitive member and electrostatic information recording medium causes changes in the field strength and hence the discharge current. This results in a change in the amount of charges accumulated on the insulating layer even in the same quanity of exposure. In order to obtain the amount of charges corresponding to the exposure energy therefore, it is required to keep the gap length constant. This is why the insulating spacer 3 has been inserted between the photosensitive member and electrostatic information recording medium every the image exposure to keep the gap length constant. In order to increase recording sensitity, it is then required to increase the amount of charges formed on the insulating layer 1a in the same exposure energy and it is necessary to this end to boost the voltage applied between the photosensitive member and electrostatic information recording medium. As the voltage increases, however, there is a problem that as when dust, etc. exist between the spacer and the photoconductive layer, discharge may take place at the spacer region, causing a breakdown of the photoconductive layer that is costly.
In addition, it is very awkward to interpose the spacer between the photosensitive material and electrostatic information recording medium to keep the gap therebetween constant, since the gap length is as short as a few tens microns. As a result, it is impossible to achieve high-speed image pickup continuously. Also, when the electrostatic information recording medium--in which electrostatic charge information has been carried--are put one upon another or rolled up--in this case, they should be flexible--for storage, there is a problem that the insulating layers may come into contact with the associated substrates, causing such information carried thereon to fall into disorder.
Usually, electrode layers are provided on the overall surfaces of the photoconductive material and electrostatic information recording medium with a spacer formed as of an insulating PET film provided between them to keep a discharge gap constant. However, when high voltage is applied on the spacer region or, especially when the spacer or its wall is bruised or otherwise flawed on the surface, surface currents flow through that spacer region, doing damage to the photosensitive member or electrostatic information recording medium and so causing their discharge breakdown. Once such discharge breakdown has occurred, the photosensitive member or the electrostatic information recording medium can never be used. Thus, the prior art poses a problem in connection with the service life of the photosensitive member or electrostatic information recording medium.
The present invention seeks to provide a solution to the above-mentioned problems.